This invention relates to deflection electromagnets for charged particle devices such as synchrotrons, and more particularly to structures of superconducting coils of 180 degrees bending magnets by which a magnetic field of improved homogeneity can be produced.
Charged particle devices are becoming increasingly important not only for research but also for industrial application purposes. For example, synchrotrons are now attracting attention as light sources in the x-ray lithography field for the production of VLSI circuits. Such synchrotrons generally comprise a pair of 180 degree bending or deflection magnets. Let us first describe the overall structure of a typical superconducting deflection magnet referring to FIGS. 1 and 2, which show the perspective and the plan view of the magnet, respectively:
The deflection electromagnet 1 comprises an upper and a lower main coil 2 and 3, each being formed of a racetrack shaped coil bent into a semi-circular form. Currents flow in the upper and lower coils 2 and 3 in the same direction shown by the arrows m1 and m2, respectively, so as to produce a magnetic field perpendicular to the plane of the orbit S of the charged particles (electrons). (The direction perpendicular to the plane of the orbit is shown at Z in FIG. 1.) Thus, the electrons, travelling along the equilibrium orbit S in the direction indicated by the arrows thereon, are deflected by the magnetic field generated by the main coils 1 and 2, so as to follow the circular path along the orbit S.
In order that the electrons are deflected properly along the orbit S, it is necessary that the magnetic field generated by the coils 2 and 3 is uniform in the order of 1.times.10.sup.-4 to 1.times.10.sup.-3 along the radial direction R perpendicular to the orbit S. If the magnetic field near the orbit S is not uniform, the electron beam traveling along the equilibrium orbit S is increasingly deviated therefrom and eventually is lost when the deviation becomes so large that the beam hits the vacuum chamber wall (not shown). Thus, a magnetic field must be produced which is uniform in the direction R along the whole semicircular length of the orbit S.
The magnetic field produced by the main coils 2 and 3, however, includes quadrupole and sextupole, etc., as well as bipolar field components, so that the magnetic field varies linearly and quadratically, etc., along the radial direction R perpendicular to the orbit S. Thus, shim coils are sometimes used as correction coils for these quadrupole and sextupole field components contained in the field generated by the main coils 2 and 3. However, such shim coils, which can be easily attached to near the middle of the semicircular main coils 2 and 3 (i.e., near .THETA.=0 in FIG. 2), are difficult to attach to the end portions 2a and 3a of the main coils 2 and 3, since there is little room left there for attachment. Hence, a large error magnetic field (i.e., the field components which vary along the radial direction which is difficult to correct is generated near the end portions 2a and 3a of the main coils 2 and 3.
The error field generated near the end portions 2a and 3a of the main coils 2 and 3 consists primarily of sextupole component. Let us explain this by referring to FIG. 3 which shows the variation of the magnetic field B.sub.z along the radial direction R near the end portions 2a and 3a of the coils 2 and 3. The magnetic field generated by the coils 2 and 3 can be regarded as a composition of the fields generated by the inner and outer branches 2b, 2c, 3b and 3c of the coils 2 and 3 (see FIG. 1). Thus, the field B.sub.z is at its maximum at R=0 where the radial direction R intersects the electron orbit S. As the absolute value of R increases from zero (i.e., as the radial distance from the orbit S increases), the magnetic field B.sub.z decreases, such that when R increases beyond the radial length corresponding to the inner branches, 2b and 3b, or outer branches, 2c and 3c, of the coils 2 and 3, the field B.sub.z takes a negative value since the inner branches 2b and 3b or outer branches 2c and 3c of the coils 2 and 3 form a magnetic field directed opposite to the field generated near the orbit S. Near the end portions 2a and 3a of the coils 2 and 3, the radial separation between the inner and outer branches of the coils 2 and 3 is smaller than near the middle of the coils 2 and 3 (near .THETA.=0); hence, the negative second order, or sextupole, component becomes especially conspicuous near the end portions 2a and 3a of the coils. Thus, as shown in FIG. 3, the variation of the magnitude of the field B.sub.z with respect to R near the end portions 2a and 3a of the coils is represented essentially by a quadratic curve which has its maximum at R=0 where the radial direction R crosses the orbit S. On the other hand, the sextupole field component is negligible on the orbit S at positions far away from the end portions 2a and 3a of the coils. FIG. 4 shows the variation of the magnitude of the sextupole component (in teslas per square meters) along the orbit S, starting from .THETA.=0 degrees (at the middle of the coils 2 and 3) and ending just beyond .THETA.=90 degrees (the end portions 2a and 3a of the coils).
As pointed out above, this sextupole component, which is conspicuous near the end portions 2a and 3a of the coils and has an adverse effect on the stability of the electron beam, cannot readily be corrected by means of shim coils, since there is little room for the attachment of the shim coils near the end portions 2a and 3a of the coils.
The magnetic field generated by the coils 2 and 3 near the end portions 2a and 3a thereof contains other multipolar components as well as the predominant sextupole components explained above. FIG. 5 shows a form of the main coils 2 and 3 of the deflection magnet disclosed in Japanese patent application laid-open (Kokai) No. 63-221598, which is intended for suppressing the non-uniform or error components of the magnetic field. The side view of the magnet of FIG. 5 is shown in FIG. 6. As shown clearly in FIG. 6, the end portions 2a and 3a of the coils 2 and 3 are bent away from the plane of the orbit S (i.e., the midplane of the deflection magnet with respect to which the coils 2 and 3 are disposed symmetrically); this design is intended for improving the uniformity of the magnetic field near the ends of the coils 2 and 3. The angle .alpha. of the bent end portions 2a and 3a with respect to the plane of the orbit S is selected at 30 degrees.+-.15 degrees (i.e., from 15 to 15 degrees). (By the way, as shown in FIG. 6, the inner branch 2b and 3b of the coils 2 and 3 are nearer to the plane of the orbit S than the outer branches 2c and 3c; this design is effective in suppressing the quadrupole component, which, however, is not directly relevant to the present invention.)
The magnet design of FIGS. 5 and 6 is effective to a certain degree in enhancing the uniformity of the magnetic field; however, it still suffer from the following disadvantages. Namely, since the structure of the coils 2 and 3 are complicated, especially at the bent end portions 2a and 3a thereof, the critical current of the coils 2 and 3 at which the transition from the superconduction to the normal conduction of the coils takes place becomes smaller; thus, it becomes infeasible to produce a magnetic field of a greater magnitude which is necessary for obtaining high energy electrons. Further, the production procedures become complicated and hence the production cost is increased. It is further also noted as a disadvantage of the coil design of FIGS. 5 and 6 that, although the uniformity of the magnetic field is increasingly enhanced as the angle .alpha. of the bent end portions 2a and 3a approaches 90, the inherent difficulty in bending the superconducting coils limits the bending angle; thus, the uniformity of the field cannot be enhanced beyond a certain level.
Superconducting deflection magnets are accompanied with difficulties other than the non-uniformity of the magnetic field pointed out above. Namely, the strength of the magnetic field which acts on the superconducting coils takes its maximum value near the end portions thereof, and the maximum field acting on the coils limits the maximum current which may flow through the coils without destroying the superconductivity thereof.
FIGS. 7 through 9 show the coil structure which is effective in suppressing the maximum value of the magnetic field which acts on the superconducting coils 2 and 3; this coil structure is disclosed, for example, in A. Jahnke et al.: "First superconducting prototype magnets for a compact synchrotron radiation source in operation", IEEE transactions on magnetics, vol. 24, No. 2, pp. 1230 through 1232, March 1988.
As shown in FIGS. 7 and 8, the end portions of the upper superconducting coil 2 are each divided into three parts 2A, 2B, and 2C, separated by spacers 4 from each other; the end portions of the lower coil 2 are divided into three parts 3A, 3B, and 3C, separated by spacers 4 from each other. The sum of the widths of these divided parts is substantially equal to the width of the non-divided portions of the coils 2 and 3. The spacers 4 are made, for example, of GFRP (glass fiber reinforced plastic). The electron beams are represented at points E on the orbit S in FIGS. 7 and 8; further, the vertical projections onto the orbit S of the central positions of the divided parts of the coils 2 and 3 and those of the spacers 4 are represented by successive points S1 through S5 thereon, the overall width of the end portions of the coils 2 and 3 being represented by W.
Let us explain the necessity of suppressing the maximum field applied on the superconducting coils by reference to FIG. 10, wherein B-I (magnetic field v. current) characteristic curve C represents the typical relation between the magnetic field B (plotted along the abscissa in T (teslas)) and the maximum current I (plotted along the ordinate in A (amperes)) which may flow through a short linear superconducting material without destroying the superconductivity thereof: when the current I exceeds the level represented by the characteristic curve C, the transition from the superconduction to the normal conduction takes place. The load curve B.sub.0 of the central magnetic field, i.e., the field B.sub.0 at the representative location at which the magnetic field is utilized (that is, a point on the orbit S in the case of the magnet of FIGS. 7 through 9, which is at the center of symmetry of the magnet), represents the relation between the magnitude of the current I and the magnetic field B.sub.0 generated there. The load curve Bmax1 represents the relation between the current I and the maximum magnetic field Bmax1 applied on the superconducting coils in the case where the coil ends are divided as shown in FIGS. 7 and 8; on the other hand, the load curve Bmax2 represents the relation between the current I and the maximum magnetic field Bmax2 applied on the superconducting coils in the case where the coil ends are not divided.
As shown by the curve C in FIG. 10, the maximum current which may flow through the superconducting coils without destroying the superconductivity thereof decreases as the magnetic field applied on the coils increases. On the other hand, the maximum magnetic field applied on the superconducting coils, Bmax (Bmax1 or Bmax2), is generated at a place where the radius of curvature of the coils is small and the magnetomotive forces generated by the coils are thus concentrated. Thus, the maximum field Bmax2 acting on the coils with divided ends is generated near the points a in FIG. 8 in the case of the coils of FIGS. 7 and 8, where the curvature of the coils 2 and 3 is at its smallest. The maximum field Bmax1 acting on the coils with undivided ends is generated near the analogous points corresponding to the points a.
The operation of coils with undivided ends may be summarized as follows. Since a small current produces a large maximum magnetic field Bmax1 at the undivided coil ends, the curve Bmax1 has a smaller inclination than the curve Bmax2. At I=350 A (amperes), the load curve Bmax1 intersects the B-I characteristic curve C in FIG. 10; this means that current exceeding 350 amperes flowing through the linear superconducting material destroys the superconductivity thereof. Generally speaking, the performance of the superconducting coils deteriorates below the level represented by the characteristic curve C (which represents the characteristic of a short linear material) in the cource of production thereof. Thus, the operating point P on the load curve Bmax1 is selected at about 80 percent of the current level of the point at which the load curve Bmax1 intersects the characteristic curve C. As shown by the dotted lines extending horizontally from the operating point P and then vertically downward from the curve B.sub.0 in FIG. 10, the central magnetic field B.sub.0 generated at the operating point P is about 1.6 T (teslas).
Compared with the maximum field Bmax1, the maximum field Bmax2 acting on the coils with divided ends is suppressed, as shown by the curve Bmax2 having a larger inclination in FIG. 10. Assuming that the operating point Q on the curve Bmax2 is selected at about 80 percent of the level of the intersection point of the curves Bmax2 and C as in the case of the coils with undivided ends, the central magnetic field B.sub.0 generated at the operating point Q becomes as great as about 2.1 T, as shown by the dotted lines extending from Q.
From the above discussion, it can be concluded that the more the maximum magnetic field Bmax acting on the coils is suppressed and is thus reduced nearer to the level of the central magnetic field B.sub.0, the stronger becomes the central magnetic field B.sub.0 that can be produced without destroying the superconductivity of the coils.
Referring to FIG. 8 of the drawings, let us now explain the mechanism by which the maximum field Bmax2 acting on the coils with divided ends is suppressed. The end parts 2A, 2B, and 2C of the upper coil 2, or those 3A, 3B, and 3C of the lower coil 3, are separated from each other by inserted spacers 4, which support the electromagnetic forces acting between these parts. Thus, with respect to direction of the orbit S, although magnetomotive forces are present at points S1, S3, and S5 thereon, no magnetomotive force is present at points S2 and S4, above and below which the spacers 4 are disposed. The magnetomotive forces generated at the ends of the coils are thus dispersed, and hence the maximum field Bmax acting on the coils is suppressed. This is the mechanism by which the maximum field Bmax acting on the coils with divided ends is suppressed.
Thus, the suppression of the maximum field Bmax on the coils by means of the divided ends as shown in FIGS. 7 and 8 has the following disadvantages. When the width W of the end portions of the coils 2 and 3 is given as an imposed condition, it is ideal to divide the end portions of the coils into infinitely many parts. In reality, however, the number of division is limited (e.g., to three, as shown in FIGS. 7 and 8) by practical difficulties in the production of the coils. Although division into three parts, for example, shown in FIGS. 7 and 8, is effective in suppressing the maximum field Bmax, the suppressive effect cannot exceed a certain limit imposed by the number of division.
Further disadvantages result from the division of coil ends as shown in FIGS. 7 and 8: FIG. 11 shows the magnetic field strength B, obtained by theoretical calculations, along the orbit S near the end portions of the coils. Compared with the case of the field B1 (represented by a dotted curve) generated by the coils with undivided ends, the variation region of the field B2 (represented by a solid curve) generated in the case where the coil ends are divided is spread over a greater length along the orbit S, since the electromotive force of the divided coil ends is dispersed over a wider region along the orbit S. The variation of the field B2 generated by coils with divided ends is uneven, since the field B2 is weakened on the orbit S at positions where spacers 4 are disposed thereabove and therebelow and, hence, no magnetomotive force is present. The uneven variation of the field over a long region along the orbit S may have adverse effects on the stability of the electron beam, because a precise adjustment of the field over the long variation range of the field along the orbit S is essential for proper deflection of the electron beams.